Various wireless access technologies have been proposed or implemented to enable mobile stations to perform communications with other mobile stations or with wired terminals coupled to wired networks. Examples of wireless access technologies include GSM (Global System for Mobile communications) or UMTS (Universal Mobile Telecommunications System) technologies, defined by the Third Generation Partnership Project (3GPP); CDMA 2000 (Code Division Multiple Access 2000) technologies, defined by 3GPP2; or other wireless access technologies.
As part of the continuing evolution of wireless access technologies to improve spectral efficiency, to improve services, to lower costs, and so forth, new standards have been proposed. One such new standard is the Long Term Evolution (LTE) standard from 3GPP, which seeks to enhance the UMTS wireless network.
Another type of wireless access technology is the WiMax (Worldwide Interoperability for Microwave Access) technology. WiMax is based on the IEEE (Institute of Electrical and Electronics Engineers) 802.16 Standard. The WiMax wireless access technology is designed to provide wireless broadband access.
In a wireless access network, a mobile station can be handed over from a source base station to a target base station as the mobile station moves between different coverage areas. Traditionally, in a WiMax wireless access network, the mobile station performs a ranging procedure during initial network entry or network re-entry or during handover, in which a randomly selected ranging code is sent by the mobile station to a serving or target base station to allow for the mobile station to acquire time synchronization with the base station. After successful reception of the ranging code, the base station allocates required resources to the mobile station to allow the mobile station to send a ranging request message to identify the mobile station's service authorization. Typically, the ranging procedures performed by multiple mobile stations are contention-based such that there is a possibility of collisions of the ranging codes sent by the multiple mobile stations. Collisions can lead to delays in the handover procedure.
To reduce latency during a handover from the source base station to the target base station, a fast-ranging handover technique is provided by WiMax. Fast-ranging handover is initiated when the target base station sends a fast-ranging information element to the mobile station. The fast-ranging information element contains the necessary resource allocation for the mobile station to transmit the ranging request message without the need of sending a ranging code. The fast-ranging handover procedure avoids contention-based ranging during the handover procedure, which reduces latency and improves handover performance. To support fast-ranging handover, WiMax defines the concept of an action time, which is the earliest time (expressed in terms of frames) from a predefined message (sent to the mobile station) at which the target base station can allocate uplink resources to the mobile station. At the action time, the target base station is able to send a message to allocate uplink resources to the mobile station, where the message includes the fast-ranging information element. The fast-ranging information element includes information identifying a dedicated transmission opportunity to the mobile station to send a ranging request message, for non-contention based ranging.
Conventionally, action time is initially determined by the target base station, based on the load at the target base station. Basically, the action time that is specified by the target base station indicates that the target base station can accommodate the incoming mobile station after a certain number of frames in view of the current loading at the target base station. This initial action time is sent to a source base station (which is the base station currently serving the mobile station). The source base station may change the initial action time provided by the target base station, and this changed action time (which is greater than or equal to the original action time set by the target base station) can be provided by the source base station to both the target base station and the mobile station. However, conventionally, the changed action time that is communicated to the target base station may not result in optimized handover performance, and in fact, can lead to increased latency in the handover.